Right, yet in real life we see effective use of those rounds out to double that 1200 yard range.

And, of course, accuracy is only one part of effectiveness. Stopping power is also important, especially in the military role, and varies greatly between cartridges; for instance .338LM is something like 5000 ft.lbs. at the muzzle vs. .308 which is somewhere around 2900.

Regarding .50 BMG - I'm not sure there is such thing as overkill if you're one of those guys on a two-way range. ;)